Corrosion resistant steel alloy



United States Patent 3,337,331 CORROSION RESISTANT STEEL ALLOY Lars Gustaf Fredrik Ljungberg, Sandviken, Sweden, assignor to Sandvikens Jernverks Aktiebolag, Sandviken,

weden, a corporation of Sweden No Drawing. Filed Jan. 26, 1965, Ser. No. 428,207 Claims priority, application Sweden, Jan. 29, 1964, 1,059/64 12 Claims. (Cl. 75-128) ABSTRACT OF THE DISCLOSURE A steel alloy having a high resistance to corrosion, high strength and good workability and weldability consisting essentially of up to 0.15% carbon, l-22% chromium, 3-8% nickel, 1-2.8% silicon, 0-2.5% manganese, 2-4% molybdenum and altogether not more than 1.5% of carbide former selected from the group consisting of niobium, tantalum and titanium, the remainder being iron and the usual impurities, the alloy containing 40-95% by volume of ferrite and the remainder being austenite.

The present invention relates to a steel alloy with high resistance against corrosion, especially stress corrosion and pitting corrosion, and which at the same time has a high strength and good workability and weldability.

The steel alloy according to the invention can be used for manufacture of for instance welded or seamless tu-bes, strips, wires and other articles and products, which are especially suitable for use in chemical and similar industry, where corroding agents are used, and has in comparison with earlier known alloys a superior resistance against stress corrosion, for instance caused by halogen compounds, and also a substantially improved resistance against pitting corrosion. The steel has also superior strength qualities and the objects made thereof are simple to manufacture by machining and welding.

Ferritic steels are resistant against stress corrosion and have a high strength but are less resistant against general corrosion and pitting corrosion. Austenitic steels are resistant against general corrosion and, alloyed with molybdenum, against pitting. They have a low strength and bad resistance against stress corrosion. Ferritic-austenitic steels are resistant against stress corrosion, have a good resistance against general corrosion and, when alloyed with molybdenum, against pitting corrosion. It is important that the relation ferrite-austenite in a ferriticaustenitic steel is balanced properly in order to obtain the desired properties. the workability of the steel, while too little austenite weakens the desired properties, i.e., the corrosion protection.

According to the invention the steel alloy has a ferriticaustenitic structure and contains up to 0.15% carbon, l5-22% chromium, 3-8% nickel, 1-4% silicon, manganese, altogether up to 1.5% of one or more carbide formers as niobium (columbium), tantalum and titanium, and also 2-4% molybdenum, the contents of the alloy elements being mutually balanced in such a way that the steel contains 40-90% by volume of ferrite, the nest being austenite.

As above mentioned the steel according to the invention has a greater resistance against stress corrosion Too little ferrite gives diificulties with than austenitic steels. In addition hereto the strength properties are superior to those of the austenitic steels, for instance of the type containing 18% chromium, 12% nickel and 2.5% molybdenum. In comparison with previous ferritic-austenitic steels, for instance of the type containing 26% chromium, 5% nickel and 1.5% molybdenum, is obtained a very substantial improvement of the corrosion properties especially with regard to pitting, but also against solutions of nonoxidizing character (for instance formic acid, oxalic acid and sulphuric acid). Contrary to the previous ferritic-austenitic steels the tendency of embrittlement because of so-called 475 C.-brittleness within the temperature interval 425-525" C. and because of formation of sigma-phase Within the temperature interval 550-850 C. is' wholly or practically wholly eliminated.

For obtaining the advantageous properties characteristic for the objects according to the invention they should be made from a steel alloy which is quench-annealed from 900-1025 C. and has a composition within the above defined limits. Of special importance in this connection is the combination of the high content of molybdenum in the alloy and the structure characteristic for the alloy, showing a substantial, as a rule predominant part of ferrite.

As earlier mentioned the share of ferrite should be 40-95% by volume while the rest consists of austenite. Usually the share of ferrite should be more than 50% and often 55% by volume and should not exceed 90% by volume. A usually suitable value is about by volume.

The content of molybdenum should be 2-4%, and is in general chosen within the lower part of this range, the upper limit being 3.5% or often 3.2%. The lower limit can in many cases be raised to 2.2%. A convenient range for many purposes is 2.4-3%. The nickel content range amount to 3-8% and the chromium content to 15-22%, preferably 16-21%.

The alloy can contain up to 0.15 carbon, but as a rule the carbon content should be substantially lower, for instance below 0.080% and preferably below 0.030%. As a suitable example can be mentioned carbon contents of 0:008-0.030%. If the carbon content lies within the upper part of said range it is suitable to add one or more carbide forming elements as niobium (columbium), tantalum and titanium at an amount up to 1.5%, usually up to 1.0%.

The alloy contains also from 1-2.8% silicon. It has been found that silicon efliciently contributes to the desired properties of the alloy, i.e., the resistance against stress corrosion. Silicon further eliminates the tendency of temper brittleness (brittleness at 475 C.) and the inven tion makes use of a quantity of silicon which is larger than usual in similar alloys. The content of silicon can be defined by the following formula:

The chemical symbols represent the quantities in percents. This condition should -be fulfilled at the same time as the quantities lie within the defined ranges.

Silicon can to a certain extent substitute chromium as a passivating and thereby corrosion preventing element, and 1% silicon has in this respect been found equivalent to 2% chromium. The content of chromium can, however, not be lowered too much as martensite is formed instead of austenite. The substitution of chromium by silicon has Patented Aug. 22, 1967- the advantageous effect that the temper brittleness (brittleness at 475 C.) is suppressed. The further advantage is also obtained that the substitution lessens the content of chromium, thereby lessening the tendency for formation of sigma-phase.

Earlier it has been difficult to avoid formation of sigmaphase in stainless steels having a ferritic-austenitic structure and containing more than 2% of molybdenum, because the molybdenum increases the trend for formation of sigma-phase. On the other hand an increased content of molybdenum is desirable because it raises the resistance against pitting corrosion in for instance solutions containing chloride. By substituting part of the chromium by silicon it has according to the invention become possible to use a content of molybdenum which is higher than in hitherto common steels without increasing the trend for formation of sigma-phase.

As a rule the silicon content should exceed 1.2% and often 1.4% and it should as a rule be at the most 2.8%. Usually the lower limit of the silicon content is 1.2-1.4% and the upper limit 1.9-2.1%. The high content of silicon thus contributes to achieving a chromium content which is exceptionally low for a ferritic-austenitic steel, usually 16-20% chromium.

In order to obtain the desired relation between the content of ferrite and austenite, it is necessary that the defined ranges for the alloy elements are observed, especially for the molybdenum, the chromium and the nickel. This condition is, however, not sufficient but the quantities of the elements must further be balanced with relation to each other. If the content of chromium approaches the upper limit, the nickel content must also be raised and vice versa in order to obtain the desired phase relation. In case of a high content of molybdenum the content of nickel should be raised and/or the content of chromium be lowered.

The conditions for obtaining the desired ferritic-austenitic structure can be expressed more in particular by the following formula:

This condition can be further specified for stabilized and unstabilized steels by the following three formulas:

(1) Unstabilized steel:

(3) Steel stabilized with niobium (colurnbium) and/or tantalum:

The chemical symbols represent the contents calculated in percents.

The content of Ti, Nb (Cb) and/ or Ta should as above remarked not exceed 1.5%.

The content of nickel should be 3-8%, the quantity being balanced within this range in the way above defined. A too high content of nickel will cause a trend to form only austenite, and too little nickel will result in a trend to form ferrite or martensite instead of austenite.

In addition to the above mentioned elements the alloy can contain up to 2.5%, usually OJ-2%, for instance about 1.5 manganese. It can further contain up to 1% of additional elements which do not have an undesirable influence upon the properties. As an example of such elements can be mentioned vanadium and tungsten. For the rest the alloy contains iron with insignificant quantities of the impurities usually occuring in iron.

As an example of a suitable composition of the alloy for the objects according to the invention the following can be mentioned: 0.008-0.030% C, 16-21% Cr, 3-8% 4 Ni, 1.42.0 Si, 05-20% Mn, 2.0-3.2% Mo and a remainder consisting in the main of iron with usual impurities.

In the following table are given examples of some alloys within the scope of the invention. The alloys had been annealed at 975 C. during half an hour and quenched in water:

Percent O N Ti Nb Ta Si Mn Percent Cr Ni Mo a-phase, -phase, volume volume As appears from the table the alloys contained exclusively (X- and -phase and were thus free from other metallic phases which as a rule are embrittling.

All these alloys showed a very good resistance against stress corrosion at 250 C. in solutions of 0.11% of chloride. In boiling oxalic acid of 10% during 7 days they were unaffected.

Further they were practically completely unaffected by pitting corrosion in a 1% solution of chloride of potassium with pH 3 or a 7.5% solution of chloride of potassium with pH 7. It can be mentioned that corresponding tests in a solution of chloride of potassium with a ferriticaustenitic steel containing 26% chromium, 5% nickel and 1.5% molybdenum showed a strong pitting corrosion.

In the above formulas the chemical symbols represent the quantities in percents. This means, for example, that if a steel containing 18% of chromium is to be checked by aid of the formula, the number 18 is inserted instead of the symbol Cr in the formula.

In the foregoing description all percentages are by weight unless otherwise indicated.

I claim:

1. Steel alloy having a high resistance against corrosion including stress corrosion and pitting corrosion, a high strength and good machining and welding properties, characterized in, that the alloy besides iron with usual impurities consists essentially of up to 0.15% carbon, 15-22% chromium, 3-8% nickel, 12.8% silicon, 0-2.5% manganese, altogether not more than 1.5% of a carbide former selected from the group consisting of niobium, tantalum and titanium and furthermore 2-4% molybdenum, the contents of the alloy elements being determined by the formula in which the chemical symbols stand for the quantities of the elements in percents and being so balanced in relation to each other that the steel contains 40-95% by volume of ferrite, the rest being austenite.

2. Steel alloy according to claim 1, characterized in, that it contains 50-90% by volume ferrite.

3. Steel alloy according to claim 1 characterized in, that it contains about 75% by volume ferrite.

4. Steel alloy according to claim 1 characterized in that it contains 2-3.2% molybdenum.

5. Steel alloy according to claim 1 characterized in, that it contains 2.4-3.0% molybdenum.

6-. Steel alloy according to claim 1 characterized in, that it contains not more than 0.080% carbon.

7. Steel alloy according to claim 1 characterized in, that it contains not more than 0.030% carbon.

8. Steel alloy according to claim 1 characterized in, that it contains 1.2-2.1% silicon.

9. Steel alloy according to claim 1, characterized in, that it contains 0.1-2.'0% manganese.

10. Steel alloy according to claim 1 characterized in, that it contains 0.008-0.030% carbon, 16-21% chromium, 3.46.8% nickel, 1.42.0% silicon, 0.5-2.0% manganese, 2.03.2% molybdenum and a remainder which in the main wholly consists of iron With usual impurities.

11. Steel alloy according to claim 1 characterized in, that it contains insignificant quantities, at the most up to 1%, of additional alloy elements selected from the group consisting of vanadium and tungsten, which have no negative influence upon the properties of the alloy.

12. Steel alloy according to claim 1 characterized in that the contents of silicon and chromium are determined by the following formula, the chemical symbols representing the quantities in percents:

References Cited UNITED STATES PATENTS 11/1958 Mott 75128 1/196-0 Mott 75-128 OTHER REFERENCES Alloys of Iron and Chromium, v01. 2, High Chromium, Kinzel and Franks, 1940, pp. 433-437.

Neue Hutte, vol. 4, #12, December 1959, pp. 716725,

15 Tauscher et a1.

DAVID L. RECK, Primary Examiner. P. WEINSTEIN, Assistant Examiner.

REEXAMINATION CERTIFICATE (121st) [45] Certificate Issued Sep. 27, 1983 [54] CORROSION RESISTANT STEEL ALLOY [75] Inventor: Lars G. F. Ljungberg, Sandviken,

Sweden [73] Assignee: Sandvikens Jernverks Aktiebolag,

Sandviken, Sweden Reexamination Request:

No. 90/000,150, Jan. 28. 1982 Reexamination Certificate for:

Patent No.: 3,337,331 Issued: Aug. 22, 1967 Appl. No.: 428,207 Filed: Jan. 26, 1965 [30] Foreign Application Priority Data Jan. 29, 1964 [SE] Sweden 1,059/64 [51] Int. Cl. C22C 38/40; C22C 38/44 [52] US. Cl. 148/37; 75/128 R; 75/128 A; 75/128 C; 75/128 T; 75/128 W; 148/38 [58] Field of Search 75/128 C; 148/38, 37

[56] References Cited FOREIGN PATENT DOCUMENTS 146720 8/1936 Austria. 866685 8/1941 France.

OTHER PUBLICATIONS Monypenny, Stainless Iron and Steel, vol. 1, Chapman & Hall Ltd., London 1951, pp. 250, 251 & 283. Oppenheim, Industrie-Anzeiger, No. 14, Feb. 17, 1959, pp. 192-193.

Archer et a1, Molybdenum, Climax Molybdenum Company, 1951, pp. 163-177.

Primary Examiner-Peter K. Skiff [57] ABSTRACT A steel alloy having a high resistance to corrosion, high strength and good workability and weldability consisting essentially of up to 0.15% carbon, 15-22% chromium, 3-8% nickel, 1-2.8% silicon, 02.5% manganese, 24% molybdenum and altogether not more than 1.5% of carbide former selected from the group consisting of niobium, tantalum and titanium, the remainder being iron and the usual impurities, the alloy containing 40-95% by volume of ferrite and the remainder being austenite.

NO AMENDMENTS HAVE BEEN MADE To REEXAMINATION CERTIFICATE THE PATENT- ISSUED UN E 35 AS A RESULT OF REEXAMINATION, IT HAS CORROSION RESISTANT STEEL ALLOY 

1. STEEL ALLOY HAVING A HIGH RESISTANCE AGAINST CORROSION INCLUDING STRESS CORROSION AND PITTING CORROSION, A HIGH STRENGTH AND GOOD MACHINING AND WELDING PROPERTIES, CHARACTERIZED IN, THAT THE ALLOY BESIDED IRON WITH USUAL IMPURITIES CONSISTS ESSENTIALLY OF UP TO 0.15% CARBON, 15-22% CHROMIUM, 3-8% NICKEL, 1-2.8% SILICON, 0-2.5% MANGANESE, ALTOGETHER NOT MORE THAN 1.5% OF A CARBIDE FORMER SELECTED FROM THE GROUP CONSISTING OF NIOBIUM, TANTALUM AND TITANIUM AND FURTHERMORE 2-4% MOLYBDENUM, THE CONTENTS OF THE ALLOY ELEMENTS BEING DETERMINED BY THE FORMULA 23.0>CR+3SI+MO+10TI+4NB +2TA-NI-0.5MN-20C-10N>18.0 